If you’re on a budget, check out RadRails (www.radrails.org). I use it,
and am very pleased so far. But, if you have the cash to spare, Komodo
is very good too. It does have some auto-complete features.

jEdit (www.jedit.org) is a free, open source editor that’s
cross-platform
(written in Java). With the addition of a few jEdit plugins, it’s
possible
to come VERY close to the screencast in terms of text autocompletion and
such.

This page has a good introduction to making jEdit rails-friendly:

http://saimonmoore.net/permalink/jedit-for-ruby-rails-development

One handy tip that’s not on that Web page…if you install the Console
and
ProjectViewer plugins, you can easily run the rails scripts from within
jEdit. You can also do the following to link ProjectViewer and Console:

1. From the Plugins menu, pick Plugin Options.
2. In the tree view, expand Project Viewer and click on the Context

Menu node.
3. Click on the + below the menu item list to add a new item to the
context menu.
4. In the Add to Context Menu dialog, select “Plugin: Console”
5. Click on “cd to ProjectViewer selected node dir” and click OK.
6. Click OK to dismiss the Plugin Options dialog.

Then you can right-click on a folder in the Project Viewer and cause the
Console shell to cd to that directory.